---
id: "CVE-2026-19867"
kind: "vulnerability"
aliases: ["CVE-2026-19867"]
title: "Mattermost: unbounded mention keys let one user slow posts for all"
severity: "medium"
cvss: 4.3
epss: null
exploited: false
status: "open"
category: "other"
products: []
ecosystems: []
tags: ["mattermost", "other", "self-hosted"]
not_affected_if: ["default-config"]
published_at: "2026-10-16T07:17:13+00:00"
first_seen_at: "2026-10-23T21:57:12+00:00"
updated_at: "2026-10-25T04:41:12+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-19867", "first_seen_at": "2026-10-23T21:57:12+00:00"}]
---

# Mattermost: unbounded mention keys let one user slow posts for all

Mattermost does not cap the number or size of mention keys a user stores in their notification settings, and it does not bound the keyword matching run on each post. An authenticated user can store an oversized mention key list on their own profile and drive up CPU and post-processing latency for other members of shared channels.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check the running Mattermost server version (for example with `mmctl version` or the System Console) and compare it against the affected ranges: 11.10.0 and earlier in 11.10.x, 11.9.1 and earlier in 11.9.x, 11.8.5 and earlier in 11.8.x, 11.7.10 and earlier in 11.7.x.

## Fix

- Upgrade Mattermost to a release that includes the MMSA-2026-00773 fix (the sources do not name the fixed version numbers, so check the Mattermost advisory).

## Timeline

- 2026-10-23T21:57:12+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-19867

## From the sources

Mattermost versions 11.10.x <= 11.10.0, 11.9.x <= 11.9.1, 11.8.x <= 11.8.5, 11.7.x <= 11.7.10 fail to cap the number and size of mention keys stored in a user's notification properties, and to bound the quadratic multibyte-keyword matching performed for each post, which allows an authenticated user to degrade post-processing latency and consume excessive CPU on behalf of other members of any channel they share via storing an oversized notify_props.mention_keys list on their own profile.. Mattermost Advisory ID: MMSA-2026-00773

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_